Aluminum coating process



Patented Nov. 16, 1948 ALUMINUM COATING PBOCESS Marshall G. Whitfieldand Victor Sheshunoil,

Garden City, N. Y., assignors, by mesneassignments, to Faircliild Engineand Airplane Corporation, New York, N. Y., a corporation of Maryland NoDrawing. Application March 6, 1945, Serial No. 581,326

Thi invention relates to an aluminum coating process and has particularreference to the coating of copper articles with aluminum and itsalloys.

We have found that although the temperature of molten aluminum (about1218 F.) is lower than the melting point of copper (about 1929 F.), copper is readily soluble in molten aluminum applied to the unmolten coppersurface, and that this solution results in the formation of an extremelyuneven and very heavy copper-aluminum alloy at the interface, which isvery brittle and impairs the strength of the copper. Also, thebrittleness of the alloy precludes working of the aluminumcoatedarticle, because the alloy is not ductile and readilyruptures, thuscausing separation of the aluminum from the copper. The afl-lnitybetween the copper and the aluminum is so avid that the solution isdiflicult to control on a time basis and also by rapid cooling such asquenching. Nevertheless, an alloy bond'between the copper and thealuminum is necessary when the aluminum is applied directly to thecopper, in order to obtain the desired adhesion between the dissimilarcopper and aluminum metals, and therefore the aforementioned objectionscannot be avoided in present practice, thus greatly proscribing thefield of use of composite aluminum and copper articles made in thatmanner. The same condition obtains between aluminum and a high copperalloy, between a high aluminum alloy and copper, and between a highaluminum alloy and a high copper alloy, but usually to a lesser degree,depending upon the nature of the alloy in each case.

It is frequently desirable to coat articles having a copper base with analuminum or its alloys, which are subjected to rigorous treatment in useor in fabrication, so that a firm bond between them is not onlynecessary to preclude subsequent separation, but the bond must notmaterially impair the physical, thermal, or electrical properties of thecopper or the aluminum, or both. Examples of such articles are aluminumalloy-lined copper base journal bearing shells. heat-radiating orexchanging articles, such as high power vacuum tubes, engine and pumpcylinders, steam tubing, and the like, aluminum-coated copper wire, busbars and the like, which must be bent drawn, or otherwise worked beforefinishing or in use, and the like, or which are subjected to widetemperature fluctuations tending to separate the metals due to differingexpansion coefflcients, but the invention is not limited t sucharticles.

After much experimentation and resfarch, we

1 Claim. (01. 22-404) have discovered that the physical properties ofthe copper-containing metal base and the aluminum-containing metalcoating are retained without solution of the copper in the aluminum, but

nevertheless with a good bond between them, when the copper-surface tobe aluminum-coated is flrst overed with a thin skin or layer of a metalof the class consisting of nickel, iron, cobalt, manganese and chromium,or alloys of two or more of these metals. Unlike intermediate coatingsof zinc, tin and the like, the metals of the aforementioned class arenot very active in contact with the copper or aluminum but constitute abarrier precluding the very undesirable solution of the copper in thealuminum. These barrier metals also have suiliciently higher meltingpoints than the aluminum or aluminum-based alloys, so that they are notappreciably dissolved therein.

The barrier layer on the copper article need be only between about threeand about seven mils in thickness, excellent results having beenobtained with an eleetro-plated layer of about five mils in thickness.The barrier layer may be applied otherwise than by electrO-plating, aslong as it is continuous, and substantially uniform in thickness. Insome cases, barrier metal-coated copper articles may be subjected to athermal diffusion treatment at about 1400 F. to improve their adhesion,probably due to a certain amount of solution and, in some instances,alloying between the barrier metal and the copper, this treatment beingconducted in non-oxidizing or reducing atmosphere to prevent oxidationof barrier metal while so heated.

After the thin barrier layer has been applied in the manner described,the article is dipped in molten aluminum or its alloy, at a temperaturesomewhat above the melting point of the metal constituting the bath,usually not more than F. above, themaximum temperature of about 1300 F.having been found suflicient for an aluminum bath, thus precludingappreciable solution of the barrier metal. I'he article is held in the,bath until the barrier layer is heated to approx mately the temperatureof the bath, or the surface to be aluminum-coated is preheated toapproximately that temperature in a neutral or reducing atmospherebefore application of the molten aluminum, whereupon a thin film ofaluminum becomes bonded to the barrier layer. which must be clean beforedipping or other application of the aluminum thereto. In the case ofdipping, the range of immersion time in the molten aluminum-containingmetal bath is from about a fraction of a second to five or moreminlites, depending upon the time required to heat the surface to becoated to the temperature of the bath, which in turn depends on thevolume of the article. In the case of preheating the surface to thetemperature of the molten aluminum-containing metal. the time of heatingdepends upon the method of heating, e. g., high frequency electricheating for a few seconds, whereas, heating as by oven, depends uponmass and conductivity of the article. An example of a method of highfrequency electric heating is disclosed in application Serial No.524,395, flied February 29, 1944, by applicant Whitfield.

When a heavy layer of aluminum is desired, molten aluminum or aluminumbase alloy is cast or frozen upon the thin aluminum iilm on the articlewhile this aluminum film is still wet, to thereby obtain a continuousand homogeneous aluminum-containing layer of the desired thickness onthe copper article when said layer has congealed. The casting may beeffected in any known way, such as by inserting the aluminumflhnedarticle in a mold of the desired dimensions and then casting aluminumbetween the wet aluminum film on the article and the mold.

A method of freezing the thick aluminum layer I directly on the articlewithout an intermediate aluminum dipping step is disclosed in saidWhitfield application. In some cases where a relatively thin aluminumcoating or film of a few thousandths of an inch is suiiicient, thearticle may be withdrawn from the molten aluminum bath and cooledwithout employing the casting step. Congealing of the final aluminumlayer may be hastened by cooling. as by quenching in oll, by air blast,or other means. This quenching procedure will also improve the physicalproperties of the article at the alloy interface.

Aluminum and aluminum alloys and copper and copper alloys contemplatedherein are predominantly aluminum and predominantly copper, generallyconsidered as aluminum and copper base metals, respectively.

It has been i'olmd that the thin-barrier layer does not materiallyimpair the ductility of the copper or the aluminum, and billets or barsof copper which have been coated with aluminum over the barrier layer inaccordance with this invention may be drawn or rolled without sepahighcoefllcient of thermal expansion minimize the possibility of looseningof the shell, are rendered feasible when lined withaluminum-alloybearing metal according to this invention.

Similarly, copper engine'and pump cylinders, copper anodes of highpower, vacuum tubes, copper heat-exchanger tubes, and the like,requiring high thermal conductivity under widely fluctuating thermalconditions, are rendered more eiilcient thermally when aluminum-cladaccording to this invention, minimizing liability of separation of thealuminum from the barrier layer or the latter from the copper, due tothe differing coeflicients of thermal expansion.

Although the process of this invention has been described in connectionwith various copper articles and uses, the invention is not limitedthereby but is susceptible of many other uses and applications, as wellas changes within the scope of the appended claim.

We claim:

The method of coating a copper-base article with aluminum-base metal,which consists in applying to the surface of the copper-base article tobe coated a thin barrier layer of at least one of the metals of theclass consisting of nickel, iron, cobalt, manganese and chromium,heating said article under non-oxidizing conditions to light weightengine crankcases, because their about 1400 F. to cause the barrierlayer metal to at least partially diifuse into the underlying surface ofsaid article, applying an aluminumbese metal in the molten state to saidbarrier layer while the latter is heated to at least the melting pointof the aluminum base metal to form a fllm of said aluminum-base metal onsaid barrier layer, and then casting an additional coating ofaluminum-base metal in the molten state upon said aluminum base filmwhile the latter is molten, whereby the aluminum-base metal forms acontinuous bonded coating of the desired thickness on said article whenthe former has congealed.

MARSHALL G. WHITFIELD.

VICTOR SHESHUNOFF.

REFERENCES err-En The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name .Date

851,684 Monnot Apr. 30, 1907 853,716 Monnot May 14, 1907 893,932 MonnotJuly 21, 1908 1,729,848 Walker Oct. 1, 1929 1,746,987 Bennett Feb. 11,1930 1,789,979 Jones et al. Jan. 27, 1931 1,799,991 Sellick et al. Apr.7, 1931

